This invention relates to packets in which a quantity of infusible material is held in a porous envelope which can be immersed in water to prepare an infusion. It is also concerned with a method of producing such packet and with apparatus for producing the packets.
When an infusion is prepared using such a packet a quantity of infused liquid is retained in the packet, usually both by the infusible material and by the material of the packet envelope, and mechanical means have been proposed for extracting at least some of that liquid by applying pressure to the packet. In particular, it has been proposed as a convenient and hygienic solution to provide the packet with draw strings which can be manipulated to contract the packet. Examples of such packets are disclosed in U.S. Pat. Nos. 3,539,355, 3,237,550, 2,881,910 and 2,466,281.
In U.S. Pat. Nos. 3,539,355, 2,881,910 and 2,466,281 a draw string or thread passes through holes in the walls of the packet, which is thus liable to leak its contents before use. Moreover, pulling on the draw strings will create high stress concentrations at the edges of the holes, with the risk of tearing and the spillage of a quantity of the infused material into the infusion as the packet is being wrung out over it. Strengthening the walls of the packet is not a solution because of cost and because it is likely to impair the efficiency of infusion.
In U.S. Pat. No. 3,237,550 a string loop passes around the packet, the ends of the loop being held captive by a staple punched through an edge of the packet. Pulling the ends apart to shorten the loop contracts the packet. The staple must hold the ends sufficiently loosely to allow them to slide easily when the ends are pulled apart, but not so loosely that it allows the loop to slacken and slip off the other end of the packet, or the ends to slip out of the staple. It will be appreciated that close manufacturing tolerances must be maintained to ensure satisfactory use of such packets and the arrangement is thus ill-suited to economical large scale production. It is also a feature of this form of packet that the wringing action is concentrated along the centre line of the packet and the side edge regions may retain a considerable part of the liquid when the loop is tightened. A somewhat analogous arrangement is shown in U.S. Pat. No. 3,539,355, in which a stiff paper strip takes the place of the string loop. This similarly does not adapt itself to economical large scale production.
It is also known from WO91/13580, WO92/06903 and CN 93103319.5 to locate a loop of thread within a packet, an intermediate portion of the loop being anchored in a heat sealed side edge margin of the packet and the ends of the loop projecting out of the opposite side edge margin. The presence of the thread in the edge seals, and in particular in the side edge anchoring the intermediate portion of the loop, weakens the seals however. This effect is intensified by the local stresses generated on the edge seal when tension is applied to the loop ends to wring moisture from the bag consequently rupture of the bag can too easily occur and the infusion material be spilled. Furthermore, these proposals for wringable infusion packets do not disclose any way in which the packets can be reasonably produced in large quantities.